1. Field of the Invention
The present invention relates to a process for producing an electrode for a battery. In particular, the present invention relates to a process for producing an electrode for a battery, wherein an electrode active material layer can be firmly formed on both surfaces of a current collector for the electrode.
2. Description of the Background
Electrodes used in lithium ion secondary batteries are presently formed by applying a coating containing an electrode active material to both surfaces of a current collector of the electrode and drying the coating. In particular, a coating for the cathode contains a cathode active material and a binder, wherein the cathode active material is suitably dispersed therein in such a manner that it is not broken. The cathode coating is first applied to one surface of a metal foil as the electrode current collector and, after drying, it is also applied to the other surface thereof in the same manner as above, and then dried. Thus, the electrode active material layers are formed on both surfaces of the current collector for the electrode. Finally, the current collector for the electrode having the electrode active material layers formed on both surfaces thereof is cut into pieces, which is then used as the electrodes.
In the case where the coating film is formed on such a metal foil in the prior art, the adhesion between the metal foil and the electrode active material layer is insufficient to causing this problem, it has the electrode active material layer to peel off. In addressing this problem, it has been proposed, for example, to increase a resin content of an electrode coating, and to add an acid. Also, Japanese Patent Unexamined Published Application (hereinafter referred to as xe2x80x9cJ. P. KOKAIxe2x80x9d) No. Hei 2-68855 discloses and specifically describes that the adhesion is improved by using an acid.
However, when such an acid is used another problem occurs. Specifically, when the electrode active material layer is formed on one surface (hereinafter referred to as xe2x80x9csurface Axe2x80x9d) of the electrode current collector and then the same layer is formed on the other surface (hereinafter referred to as xe2x80x9csurface Bxe2x80x9d) thereof, the adhesion of the current collector to surface B is much decreased as compared with the adhesion of that to surface A. Therefore, the electrode thus prepared exhibits the peeling off of the electrode active material layer, particularly from the back surface (surface B) of the electrode current collector. When such peeling occurs, the capacity of the battery produced therefrom is lowered, or the electrode active material layer thus peeled off is interposed between a separator and, for example, a cathode electrode, and it breaks the separator to cause the short circuit of the cathode electrode and the anode electrode, disadvantageously. Such a product is, therefore, not practically usable as a battery element.
Accordingly, it is an object of the present invention to provide a process for producing an electrode for a battery by successively applying an electrode coating containing an electrode active material, a binder, a solvent and an acid to both surfaces of an electrode current collector to obtain excellent adhesion between the electrode active material layer and the electrode current collector and to avoid peeling-off of the electrode active material layers from both surfaces of the electrode current collector.
In particular, the above object and others are provided by a process for producing an electrode for a battery by successively applying an electrode coating containing an electrode active material, a binder, a solvent and an acid to both surfaces of an electrode current collector, wherein after applying the electrode coating to one surface of the electrode current collector and drying it, the other surface thereof is cleaned with water prior to the application of the electrode coating thereto.
The present invention is predicated upon the surprising discovery that the above-described object can be effectively attained by a process for applying an electrode coating containing an electrode active material, a binder, a solvent and an acid to both surfaces of an electrode current collector successively, wherein after applying the electrode coating to one surface of the electrode current collector and drying it, the other surface thereof is cleaned with water prior to the application of the electrode coating thereto. The present invention has been completed on the basis of this finding.
In more detail, the electrode coating used in the present invention contains an electrode active material, a binder, a solvent and an acid.
The electrode active materials usable herein are not particularly limited and those used hitherto as the electrode active materials are usable.
The materials used as the electrode active materials are various, and they are selected suitably depending on the use for the cathode or anode. Carbonaceous materials are those usually used as the cathode active materials. The carbonaceous materials are those used hitherto and not particularly limited. They are, for example, amorphous carbon, acetylene black, petroleum coke, coal coke, artificial graphite, natural graphite, graphite carbon fibers and difficultly graphitizable carbon.
The anode active materials are those used hitherto. Various cathode active materials, are usable without particular limitation. Various cathode active materials, such as lithium cobaltate and lithium manganate are usable.
The electrode coating in the present invention contains usually about 10 to 75% by weight, based on the solid content of the electrode coating, and preferably about 25 to 55% by weight, of the electrode active material.
Any binder may be useable so long as it is conventionally used in this technical field, without particular limitation. The binder includes, for example, polyacrylonitrile (PAN), polyethylene terephthalate, polyvinylidene fluoride (PVDF) and polyvinyl fluoride.
The binder is used in an amount of about 1 to 40 parts by weight, preferably about 2 to 25 parts by weight, and particularly about 5 to 15 parts by weight, per 100 parts by weight of the electrode active material.
The solvent is not particularly limited and any of those used hitherto for the preparation of electrode coatings can be used. The solvent includes, for example, N-methylpyrrolidone (NMP), pyrrolidone, N-methylthiopyrrolidone, dimethylformamide (DMF), dimethylacetamide and hexamethylphosphamide. They are used either alone or in the form of a mixture of them.
The solvent is used in such an amount that the solid content (nonvolatile matter content) of the electrode coating is about 10 to 80% by weight, preferably about 30 to 60% by weight and particularly preferably about 35 to 45% by weight.
The acid may be either an organic acid or an inorganic acid. As the acids, weak acids are preferred, and weak organic acids are particularly preferred. Preferred examples of the weak organic acids include oxalic acid, formic acid and maleic acid, and hydrates of these acids.
The acid is used in an amount of usually about 0.001 to 5 parts by weight, preferably about 0.01 to 3 parts by weight, per 100 parts by weight of the electrode active material.
When the electrode active material has a low electric conductivity, an electric conductor can be used, if necessary. As the electric conductors, the above-described carbonaceous materials are usable. In this cases the electric conductor is used in an amount of usually about 1 to 25 parts by weight, preferably about 3 to 15 parts by weight and particularly preferably about 5 to 10 parts by weight, per 100 parts by weight of the active material.
Metal foils are preferably used as the electrode current collectors in the present invention. The metal materials for the electrode current collectors are not particularly limited, and various metal materials used hitherto for this purpose are usable. Such metal materials are, for example, copper, aluminum, stainless steel, nickel and iron.
The electrode coating used in the present invention is prepared by mimed the above-described ingredients together, and it is in the form of a slurry. The electrode active material must be suitably dispersed in the electrode coating to such an extent that it is not broken. The mixing and dispersion are conducted by means of a planetary mixer, ball mill or the like.
The electrode coating is applied to both surfaces of the electrode current collector and dried to form the electrode active material layers.
The electrode coating can be applied to the electrode current collector by a well-known method such as extrusion coating, gravure coating, reverse roll coating, dip coating, kiss coating, doctor coating, knife coating, curt coated or screen printing.
The electrode coating thus applied is dried by a method which varies depending on the kind of the solvent. For example, it can be dried with hot air of a temperature of 80 to 300xc2x0 C.
In the present invention, the electrode active material layer is formed by applying the electrode coating to one surface of the electrode current collector and then dried under the above-described conditions. Before the electrode coating is applied to the back surface (the other surface, surface B) of the electrode current collector and dried, this surface to be coated must be cleaned with water. By this cleaning operation, the adhesion of the electrode active material layer to the back surface (surface B) of the electrode current collector can be remarkably improved Water used for the cleaning may be any of pure water (distilled water), ion-exchanged water, tap water, industrial water, well water, etc. Among them pure water and ion-exchanged water having a low impurity content are preferred.
The electrode current collector can be cleaned by, for example, a method which comprises winding a cloth around a roller, impregnating the cloth with water and rotating it to bring it into contact with the current collector; a method which comprises winding a cloth around a rod, impregnating the cloth with water and reciprocating the rod widthwise in contact with the electrode current collector; or a method wherein water or water vapor is directly jetted to the electrode current collector. Other embodiments of the cleaning method are obvious to those skilled in the art. However, in immersion methods such as a dipping method, bad effects may be exerted, such as falling-off of the dried coating layer and, therefore, the layer must be carefully protected by, for example, masking.
For facilitating the drying operation after the cleaning, a mixture of water and an organic solvent may be used for the cleaning so far as the cleaning effect is not reduced. The organic solvents usable herein are those highly soluble in water such as methanol, ethanol and acetone.
The amount of the organic solvent is usually 0 to about 60% by weight, preferably 0 to about 50% by weight, based on the mixture.
The cleaning temperature is usually about 5 to 50xc2x0 C., preferably about 10 to 40xc2x0 C. When this temperature is excessively low, the defining eject is reduced and, on the contrary, when it is excessively high, the foil is possibly denatured after the cleanup. When the temperature is kept in this range, water vapor can be used in place of water. The term xe2x80x9cwaterxe2x80x9d thus indicates herein not only water but also water vapor.
When the electrode coating is applied to the back surface of the electrode current collector, the surface is preferably dry. Therefore, the water is wiped off with a dry cloth or air is blown against the electrode current collector to dry it.
The thickness of the electrode having the electrode active material layers on both sides of the current collector can be controlled, if necessary, by roller pressing or the like.
The electrode material thus obtained is then cut into pieces having predetermined width and length. Preferably, a part of the electrode current collector is left free from the electrode active material layer so as to provide electric contact with the outside. The material partially lacking the electrode active material layer can be formed by, for example, a method wherein an uncoated pant is formed in the coating step or a method wherein the electrode active material layer is once formed and then a part thereof is removed.
The adhesion of the electrode active material layer, formed at first on one surface (surface A) of the electrode current collector to this surface A is higher that of the electrode active material layer, formed on the other surface (surface B) thereof, to this surface B. The reasons why the adhesion to the surface B is inferior to that of the surface A have not been elucidated yet. Although it has not been theoretically proved yet, the mechanism is supposed to be as follows: The acid contained in the coating is evaporated during the drying and adheres to the surface B of the electrode current collector to form some compound with a metal of the electrode current collector. Although it was considered to solve such a problem of the poor adhesion, by differentiating the ratio of the electrode active material to the binder on the surface A from the ratio on the surface B, such a method causes a problem that the varied of the electrode coatings to be prepared is increased in number, and the steps in the production process age increased to make the process complicated. The present inventors have surprisingly discovered that the cleaning of the surface B with water after the coating of the surface A and before the coating of the surface B of the current collector is very effective in efficiently improving the adhesion without necessitating such complicated steps.